Multi-Modal "Order-Order" Kinetics in Ni 3 Al Studied by Monte Carlo Computer Simulation
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Multi-Modal "Order-Order" Kinetics in Ni3Al Studied by Monte Carlo Computer Simulation P. Oramus, R. Kozubski, V.Pierron-Bohnes1, M.C.Cadeville1, W.Pfeiler2 Institute of Physics, Jagellonian University, Reymonta 4, 30-059 Kraków, Poland. 1 Institut de Physique et Chimie des Matériaux de Strasbourg, 23, rue du Loess, 67037 Strasbourg, France. 2 Institut für Materialphysik, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria. ABSTRACT “Order-order” relaxations in γ'-Ni3Al previously extensively studied by means of resistometry, are simulated within a model of vacancy mechanism of atomic migration in a superstructure implemented with Monte Carlo technique and the Glauber algorithm. The observed operation of two simultaneous relaxation processes showing different rates, as well as the theoretically predicted effect of vacancy ordering have been definitely reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing L12-type superstructure shows that the experimentally observed features of the "order-order" processes in Ni3Al follow from an interplay between two dominating and coupled modes of long- and shortrange ordering: the creation/elimination of nn pairs of antisites (SRO) and the change of the “overall” number of antisites (LRO). High profile of the first process results in a high contribution of the fast component of LRO kinetics. INTRODUCTION If a long-range ordered system annealed at temperature Ti is abruptly cooled-down or heated-up to temperature Tf, the degree η of its long-range order (LRO) - i.e. the number of antisite atoms, evolves from the initial equilibrium value to the final one. If both Ti and Tf temperatures are lower than the "order-disorder" transition point, the process is called an "orderorder" relaxation. The present Monte Carlo simulation study concerns "order-order" relaxations in a homogeneous A3B binary system with a superstructure of L12 -type and refers to the previous experimental works on Ni3Al [1]. The interest is focused on the origin of the complex character of the η(t) relaxation isotherms measured by means of residual resistometry, which fitted weighted sums of exactly two single exponentials with substantially different relaxation times. Preliminary results of the study have been presented on MRS Spring Meeting in 1998 [2]; an extended paper containing the complete material will appear soon [3].
MONTE CARLO SIMULATION PROCEDURE The sample was simulated by arranging two kinds of atom (Ni and Al) taken in a stoichiometric proportion NNi / NAl = 3/1 over an L12-type superlattice containing 256000 sites. Subsequently, a fixed number of 10 vacancies were introduced by emptying at random the corresponding number of lattice sites.
N5.10.1
Following the tradition of the pioneering works of the sixties [4,5], the equilibrium configuration of the system corresponding to a given temperature was then generated by imposing periodic boundary conditio
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